This project is concerned with certain features of the maternally inherited condition referred to as "sex-ratio", SR, in some neotropical species of Drosophila. SR is a consequence of transovarial transmission of micro-organisms (SR-spirochetes) from generation to generation and is phenotypically expressed as the absence of males in the adult progeny of SR females. Lack of males is a consequence of eary zygote mortality through action of a toxic agent produced by the SR-spirochetes. Current research is directed toward a more detailed understanding of the nature of the SR-spirochetes and viruses associated with them, as well as the basis for the susceptibility of male and the resistance of female zygotes to toxic action. SR strains in four different species distributed over northern South America and Caribbean Islands are included in these studies.